This project is designed to provide information about the organization of neuronal systems in the mammalian spinal cord that are involved in the neural control of movement. There are three current sub-projects: 1) studies of synaptic transmission; 2) studies of the neural circuits that produce coordinated locomotion in a primitive amphibian; and 3) studies of the quantitative morphology of motoneurons in the neonatal mouse spinal cord. All of these studies use in vitro preparations of brain stem slices or isolated en bloc spinal cord of neonatal mice or amphibians. Analysis of data from in vitro studies of low-frequency synaptic depression of group Ia monosynaptic excitatory postsynaptic potentials (EPSPs) produced in lumbosacral spinal motoneurons by repetitive stimulation of dorsal roots in neonatal mice was completed in FY2004. The final stage of this analysis dealt with the effects of changing external calcium concentration, application of calcium channels blocking agents, and substitution of strontium for calcium on low-frequency synaptic depression at two postnatal ages, P2-4 and P10-12. We used an empirical synaptic release model described in previous Annual Reports to show that low external calcium reduces the size of the presynaptic pools of readily-releasable transmitter and/or release-ready release sites in both young and older mice. Furthermore, two activity-dependent presynaptic processes that enhance transmitter release exhibited markedly reduced sensitivity to calcium influx in the older age group as compared to the younger. This was a surprising finding because both processes have been assumed to be calcium-dependent. A research report on these findings is being revised for resubmission to the Journal of Neurophysiology. The long range goal of a second project is to elucidate spinal cord mechanisms that control rhythmic walking movements in a relatively primitive amphibian, Necturus maculosus. In order to design appropriate electrophysiological experiments to examine the spinal circuits involved in locomotor movements in Necturus, we completed an extensive study of the anatomy of the cervical spinal cord in Necturus, which had never before been described. The morphology of motoneurons, interneurons, and primary afferents were labeled with fluorescent tracers and studied by confocal microscopy. These data were supplemented by ultrastructural analysis of specific regions with electron mcioscopy. A complete report of this work has been published in the Journal of Comparative Neurology. We have completed an extensive analysis of data from 17 fully-reconstructed lumbosacral motoneurons in the same two age groups (P2-4 nd P10-12) of neonatal mice that were obtained during the above electrophysiological experiments. The analysis was done in collaboration with Dr. Giorgio Ascoli at the Krasnow Institute of George Mason University. Despite considerable growth in overall size, especially dendritic length, motoneurons in the two age groups showed little change in the branching structure (topology) of their dendritic trees. A full report has been accepted for publication in the Journal of Comparative Neurology.